First slide

Phasors and Phasor Diagrams

Question

Which one of the following curves represents the variation of impedance (Z) with frequency f in series LCR circuit

Moderate

A
B
C
D

Solution

$Z = \sqrt{R^{2} + {(2 π f L - \frac{1}{2 π f C})}^{2}}$
From above equation at $f = 0_{\Rightarrow z = \infty}$
When $f = \frac{1}{2 π \sqrt{L C}}$ (resonant frequency) $\Rightarrow Z = R$
For $f > \frac{1}{2 π \sqrt{L C}} \Rightarrow$ Z starts increasing.
i.e., for frequency 0 – $f_{r}$ , Z decreases
and for $f_{r} t o \infty$ , Z increases. This is justified by graph in third option

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